For home and small office telephone exchange, two types of equipments are generally made available. One option is the non-Key System Unit (KSU-less) Telephone sets with integrated intercom capability carried out over the very in-house telephone line. Another option is the small Key System Unit which, albeit less costly, requires from the user technical expertise in order to install it and put it in inoperation, contrarily to KSU-less Telephone sets which are plug-and-play devices. In addition, mosts users of small telephone exchange equipments generally face other types of communication needs such as call recording, room monitoring, music on hold, busy-line indication and security control, most of which call for distributed accessories throughout the location. The disclosed inventive system makes available a low-cost KSU system with plug-and-play characteristics (Unit may be plugged anywhere at the location and requires no more than plug-in installation of the extension telephone sets) and provides dedicated interfaces for most of the secondary communication and security control functions.